1. Field of the invention
This invention comprises apparatus and method for controlling gauge uniformity in the manufacture of blown plastic film.
2. Review of the Prior Art
Production of blown plastic film by the processes of extruding molten polymer through an annular die nozzle, surrounding the extruded tube by an air cooling ring, cooling the extruded tube by discharging air from the air cooling ring onto the surface of the extruded tube, holding the extruded polymer tube closed between opposed nip rolls, inflating the closed tube between the annular die nozzle and the nip rolls to a diameter greater than that of the annular die, and drawing the polymer tube from the nozzle with the nip rolls at a rate greater than the linear rate of discharge from the annular die nozzle is well known. Uniformity of the nozzle gap, distribution of the air from the cooling ring, uniformity of temperature and attendant viscosity of the molten polymer, cleanliness and maintenance of the equipment and environmental factor in the building in which the equipment is located have all been recognized as influencing the uniformity of the gauge of the finished blown film. In view of the detrimental effects of gauge non-uniformity to subsequent processing wound rolls of blown film, it is customary to rotate portions of the equipment, e.g. extruder, die, nip rolls and collapser, to effect a distribution of gauge non-uniformity across the roll face.
While the previously mentioned factors have long been recognized to lead to gauge non-uniformities in the finished film, numerous schemes have been proposed to deliberately adjust some of these same factors to compensate for the gauge non-uniformities or to produce a desired thickness profile. In the flat die area, both local mechanical adjustments of the die opening in response to a thickness measurement signal, U.S. Pat. Nos. 3,122,782 and 3,122,783 and direct local heating of selected portions of the die responsible for greater thickness U.S. Pat. No. 3,161,711 are known.
A series of U.S. Pat. Nos. 4,246,212; 4,339,403; 4,339,404; 4,351,785; 4,425,290; 4,464,318 extend the basic methods of mechanical deformation and/or temperature variation of the die lips to annular blown film dies. In U.S. Pat. No. 4,246,212 the outer nozzle ring of the annular die has been subdivided into a number of temperature control sectors which can be separately supplied with heated or cooled fluids to locally affect melt viscosity and the resulting film thickness.
In U.S. Pat. No. 4,339,403 the correcting sections of the nozzle ring, either mechanical or thermal, are influenced in response to the circumferential lengths of film sectors of constant cross sectional area, until such circumferential lengths are equal. The remaining patents in this series deal with methods for determining and implementing correction commands.
In European EPO 325 961 A2 blown film thickness control by means of infrared heaters arranged about the bubble circumference downstream of the die and air ring has been shown. In this method, infrared heaters, whose wavelengths must be matched to the absorption characteristics of the film being processed, are used to heat locally thick areas, thereby reducing the resistance to deformation, with the net result that the thickness uniformity following transverse expansion is improved. Use of heaters in the flat die area in cases of biaxial orientation involving reheating followed by machine direction and transverse deformation are generally known, as described in U.S. Pat. No. 3,782,873.
While the previously mentioned patents all describe variation in techniques previously used in flat die extrusion applications, U.S. Pat. No. 4,209,475 describes an apparatus and method for controlling film thickness that is unique to blown film processing. As previously indicated, in the blown film process it is customary to apply one or more stream of cooling air to the extruded tube following extrusion. In the '475 patent an air ring is described that is equipped with a large number of independently adjustable blades that can project into the discharged cooling air stream. These deflector blades, by locally inducing more or less local turbulence into the cooling air stream, will offset cooling of the extruded tube along longitudinal bands. By selectively altering the cooling rates along the longitudinal bands in response to gauge variation in the expanded film, these variations can be reduced, i.e. slower cooling of thick bands relative to adjacent thin ones results in proportionally more deformation during transverse tube expansion and consequent variation or difference in film gauge. Although this patent primarily dealt with manual actuation of the deflector blades, mention was made of a system utilizing motorized adjustment of individual blades in response to control signals from an online thickness measuring device.